The present invention relates to the generation of power in general and, more particularly, to a novel control system for obtaining maximum power output from an electrical generating fluid-driven prime mover having a velocity-dependent power output for driving a generator, alternator or the like. An example of a prime mover of the type described is a windmill. Another example is a water wheel.
A theoretical and empirical analysis of windmills, a similar analysis being applicable to water wheels and other fluid-driven prime movers, reveals that the power output, P, of a windmill is directly proportional to the radius, R, of the blades, the velocity, V, of the wind, the mass density, .rho., of the air in the wind, and a power coefficient, P.sub.c, according to the relationship EQU P .varies. P.sub.c .rho.R.sup.2 V.sup.3 ( 1)
the power coefficient, P.sub.c, is also a function of wind velocity. More particularly, it is a function of the geometrical arrangement of the windmill, and of the tip speed ratio 2.pi.nR/V where n is the angular velocity of the blades, and R and V are as previously defined. The power coefficient, P.sub.c, has been determined in wind-tunnel tests for various blade arrangements. From the foregoing analysis and the tests referred to, it is found that for a given windmill there is an optimum angular velocity of the blades at which maximum power is obtained. For maximum efficiency of the system, and hence the generation of maximum electrical power over a useful range of wind velocities, it follows therefrom that the angular velocity of a windmill should be maintained at that velocity for which maximum power is obtained, regardless of any change in wind velocity.
Heretofore, a number of schemes have been proposed and employed for controlling the angular velocity of a windmill for most efficient power production. In electrical generating systems, for example, it has been the practice to vary either the load on the generator or to vary the field current or both as a function of the wind velocity for the purpose of loading the driving windmill.
For example, in a typical system employing load control, a wind switch comprising a spring-biased wind vane or wind-driven fan or the like has been employed for selectively varying a load, such as a plurality of batteries, coupled to the generator.
The switch, in response to predetermined wind velocities opens and closes electrical circuits for removing and adding electrical loads from and to the generator. In use for charging batteries, for example, the switch at low wind velocities couples a first predetermined number of batteries to the generator and at higher wind velocities couples an additional number of batteries to the generator. For all practical purposes, it has not been possible to obtain any reasonably close correspondence between the electrical power generated and the output power of a windmill over any reasonably wide range of wind velocities using such methods.
More practical than load control in most electrical applications is generator control. Generator control typically employs a means for controlling the excitation field current, and hence the electrical power output.
In such a system, the resistance of the field circuit is controlled to vary the current in the field circuit. In a number of these sytems, for example, a variable resistance, such as a carbon pile resistor is employed. A mechanical linkage, belts, pulleys and the like are used for detecting changes in the velocity of the windmill and a mechanism in response to such changes controls the pressure on the resistor and thereby its resistance.
In another such system, a wind switch, like that described above which has been used in load control systems, is employed for controlling the placement of a number of discrete resistors in the field circuit. With changes in wind velocity, a corresponding change is made in the number and arrangement of the resistors.
Even with field control, however, prior known winddriven electrical generating systems are still quite inefficient over a wide range of wind velocities because of the difficulty in building a control system with a third power dependence on wind velocity.
Theoretically, at least, it would appear to be possible to come somewhat closer to the power output curve than has been possible in the prior known wind-driven electrical generating systems by expanding the techniques employed in those systems, but the number of resistors, switches and relays that would be required would be costly and wasteful of energy.
A further characteristic of winds which is of interest is the distribution of wind velocities over a period of time.
Winds are ordinarily classified in two groups: prevalent winds and energy winds. In wind studies it is found that prevalent winds blow about 75 percent of the time and energy winds blow about 25 percent of the time. Energy winds, however, have average velocities about 2.3 times those of prevalent winds and, therefore, contain about 75 percent of the total energy generated by winds.
Since, theoretically, a maximum of only 59.2 percent of the power in the wind is obtainable by a windmill to do work, and that in practice only 30 to 40 percent is actually obtained using the most efficient windmills presently known, it is extremely desirable to not only have a more efficient system but one which is efficient over a wide range of wind velocities. Yet, it is known that conventional windmills employed in the generation of electrical energy, typically operate only at relatively low wind velocities and, therefore, do not take advantage of the energy available at higher wind velocities.